Focus on network of hidden faults

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A network of hidden faults on the outskirts of Christchurch will be the focus of more research to ascertain their earthquake risk.

Seismic surveying by Canterbury University and Calgary University researchers has revealed 10 previously unknown faults west and southwest of the city, including one running from near Lincoln to Halswell.

The survey work is looking at the area between the ends of the Greendale and Port Hills faults, known as "the gap".

At a briefing yesterday, GNS Science natural hazards platform manager Kelvin Berryman said a 30 kilometre line had been surveyed from north of Lincoln across State Highway 1 near Weedons to State Highway 73 just east of West Melton

A cross-section of rocks and sediment to about 1.5km underground showed five faults between about Lincoln and Weedons extending to within about a hundred metres of the surface.

Between there and West Melton there was also evidence of five very old faults in rocks below about 1km, he said.

The surveying had shown there was no obvious connection between the Greendale and Port Hills faults, although the Greendale Fault might extend further east than previously thought.

A fault running from west of Lincoln to northwest of Halswell ran through the "gap" and separated the two other faults.

The magnitude-5.4 earthquake on June 21, centred near Halswell and Prebbleton, was the strongest locally felt earthquake of the 10-month series.

With ground accelerations of up to 60 per cent of gravity, it was felt more sharply than the September 4 magnitude-7.1 shock, Berryman said.

"In the Halswell-Lincoln zone so far, the total amount of energy released on that [fault] is less than that of a magnitude-6.0 earthquake, so that could be one of the candidates if there were to be another magnitude-6.0-plus earthquake.

"We need to be careful and cautious.

"This is quite preliminary. There's no evidence of these things at the surface. It may have done its dash, but there's still a lot of activity going on over there," Berryman said.

"That is a good place to keep working for a few months."

The Boxing Day quake, generated by a small fault under the northern central business district, was still a mystery, he said.

"There has been no further activity on that, which I'm guessing is a really good sign."

The June 13 quake appeared to have been produced by a six to eight-kilometre fault roughly parallel to the coast from north of New Brighton to the Avon-Heathcote Estuary and possibly further south.

GNS Science has updated computer-generated quake forecasts for the Canterbury aftershock zone, saying there is a 23 per cent chance of a magnitude-6.0 to 7.9 quake in the next 12 months.

If that does not happen, that will fall to a 20 per cent chance on August 15 for the subsequent year, and, similarly, to 18 per cent on September 15 for the following 12 months.

The probability of a magnitude-5.0 to 5.9 in the next year remains high, at 94 per cent.

Berryman said for every month that passed without a substantial quake, the probabilities dropped.

"There will still be further aftershocks. The chance of a magnitude-5.0 to 5.9 aftershock hangs in there with a very high likelihood of happening – it's almost certain. We need to be prepared for those.

"Sooner or later they are going to stop, but that's going to be years away. It would be unrealistic for that to just stop."